CH627406A5 - METHOD FOR ADJUSTING THE REPRODUCTION OF AN IMAGE PLAN. - Google Patents

Description

The invention relates to a method for precisely adjusting the reproduction of an image original, in which the image original spanned on a scanning cylinder is scanned point by point and image line by line by an optoelectronic scanning element in order to obtain an image signal, and in which the reproduction on a recording cylinder is controlled by a recording element controlled by the image signal is carried out, a circumferential mark determining the start of scanning being provided on the scanning cylinder.

From DE-OS 2 513 042 a method for setting the exact reproduction of an image original by means of an engraving system is already known, in which the desired start of scanning on each image line of the original is defined by an initial mark on the scanning cylinder. The scanning begins whenever this starting mark is just under the scanning element. The circumferential position of the scanning member relative to the scanning cylinder is identified by a fixed reference mark.

A “start of scan” signal, which is obtained from the circumferential pulse of the cylinders by means of an adjustable time delay, starts a sampling clock sequence with which the sampling of the pixels, the digitization of the image signal and the intermediate storage of the digital image signal are controlled.

The start of scanning is set in such a way that, during a test run, the marks are matched by changing the time delay when the "Start of scanning" signal appears.

A stroboscope triggered by the “start of scanning” signal is used as the observation device. The start of scanning is determined when the marks are in cover at the time of the flash of light.

The scanning in each image line is interrupted by switching off the sampling clock sequence by means of a signal “scanning end”, which is created by counting the scanned pixels per image line.

In the case of seamless engraving, there is another end mark on the scanning cylinder that determines the scanning end on each image line, and the stroboscope is also triggered by the “scanning end” signal, which creates a second flash of light. By changing the frequency of the scanning clock sequence, the end mark and the reference mark of the scanning element are brought into register at the time of the second flash of light.

The start of engraving on the printing cylinder is determined by a “start of engraving” signal that starts a recording cycle in a defined phase. The "start of engraving" signal is generated by an adjustable time delay from the "start of scanning" signal.

When engraving, you assign a further starting mark to the desired start of engraving on the printing cylinder and a fixed reference mark to the circumferential position of the recording element relative to the printing cylinder.

Both marks are to be brought into coincidence by specifying the start of engraving by means of a stroboscope triggered by the “start of engraving” signal.

Reference marks are required in all applications5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

3rd

627 406

lent. These reference marks must be arranged very precisely on axially parallel lines to the cylinders, which run through the point of incidence of the optical axis of the scanning or recording element, which proves to be extremely disadvantageous.

The invention is therefore based on the object of specifying a method in which no reference marks are required and a more precise setting takes place at least at the beginning of the reproduction.

To achieve this object, the method according to the invention of the type specified at the outset is characterized in that a first signal is generated by scanning an initial mark that determines a desired start of scanning on the image template, and that a second signal “start of scanning” is derived from a circumferential pulse by a variable time delay device, which is generated by the circumferential mark at a certain angular position of the scanning cylinder, and that in order to set the start of scanning on the image template for later reproduction, the signals are matched during a test run by means of a comparison device when the signal "start of scanning" appears by changing the time delay device .

Embodiments of the invention are explained below with reference to the drawing. The drawing shows:

1 is a basic block diagram of an engraving system with a comparison device,

2 shows an application example of the comparison device when setting the start of scanning,

3 shows an application example of the comparison device when setting the start of scanning and engraving;

4 shows an application example of the comparison device when setting the scan start and scan end,

Fig. 5 shows an embodiment of the comparison device and

Fig. 6 shows another embodiment of the comparison device.

1, a scanning cylinder 1 and a printing cylinder 2 of an engraving system, not shown, are connected to one another via a coupling 3. The scanning cylinder 1 and the impression cylinder 2 are driven by a synchronous motor 4 in the direction of arrow 5. The synchronous motor 4 is fed via a converter 6 from a network 7 with the frequency fj. The converter 6 generates an art network 8 from the network 7, the frequency f2 of which depends on the frequency of a guide clock sequence T! of the converter 6 is dependent. The speed of the synchronous motor 4 is proportional to the frequency f2 of the art network 8 and thus also the frequency of the guide clock sequence Tj.

In the exemplary embodiment, the guide clock sequence Tx is obtained by frequency division from a clock sequence T0 of a generator 9. For this purpose, a first divider stage 10, whose divider factor qi is adjustable, and a second divider stage 11 with constant divider factor q2 are connected between generator 9 and converter 6. By setting the divider factor qa of the divider stage 10, the frequency of the guide clock sequence Tj and thus the speed of the synchronous motor 4 or the speed of the scanning cylinder 1 and printing cylinder 2 can be varied.

An image original 12 is precisely positioned on the scanning cylinder 1 with the aid of a series of register pins 13, which are arranged on a surface line 14 of the scanning cylinder 1.

To obtain an image signal, the image original 12 is optoelectronically scanned by a scanning element 15 image line by image line. The scanning member 15 can be moved parallel to the scanning cylinder 1 in the direction of arrow 16 by a feed device, not shown.

To digitize the analog image signal, an A / D converter 17 is provided, the analog input 18 of which is via a

Amplifier 19 is connected to the sensing element 15. The digital outputs 20 of the A / D converter 17 are connected to the data inputs 21 of an image line storage device 22 with a write address counter 22a, a read address counter 22b and a digital memory 22c.

A sampling clock sequence T3 with the frequency f3 is used to control the sampling, the analog-digital conversion of the image signal in the A / D converter 17 and the writing of the data into the digital memory 22c.

The sampling is initiated by defined starting of the sampling clock sequence T3 and ending by stopping the sampling clock sequence T3.

The sampling clock sequence T3, which is obtained by frequency division from the clock sequence T0 of the generator 9 in a divider stage 23, whose division factor q3 is adjustable, passes through an input 24 of a synchronization stage 25 to an AND gate 26 and via its output, the one output 27 corresponds to the synchronization stage 25 for selecting the write addresses on an input 29 of the write address counter 22a and via a line 30 to a control input 31 of the A / D converter 17 for controlling the analog-digital conversion.

In order to recover the image signal from the stored data, data outputs 32 of the image line storage device 22 are connected to the digital inputs 33 of a D / A converter 34. The analog output 35 of the D / A converter 34 is connected to an engraving element 37 via an engraving amplifier 36.

A recording clock sequence T4 with the frequency f4 is provided to control the reading process of the data from the digital memory 22c and to rasterize the image template 12 during engraving. The recording clock sequence T4 is also obtained by frequency division, in a divider stage 38 with a constant division factor q4, from the clock sequence T0 of the generator 9. The recording clock sequence T4 passes from the divider stage 38 via an input 39 of the synchronization stage 25 to a further AND gate 40 and via its output, which corresponds to an output 41 of the synchronization stage 25, for selecting the read addresses at an input 43 of the read address counter 22b and to rasterize the original 12 via a line 44 to the engraving amplifier 36.

In the engraving amplifier 36, the recording clock sequence T4 is converted into a sinusoidal AC voltage and this is superimposed on the image signal. The engraving element 37 with an engraving stylus as the cutting tool carries out the engraving 45 on the printing cylinder 2.

The engraving member 37 can also be moved in the direction of arrow 46 with the aid of a feed device, not shown, parallel to the printing cylinder 2.

In the engraving system described, scanning element 15 and engraving element 37 are not moved during the scanning or recording of an image line. The scanned image lines are therefore parts of parallel circumferential lines of the scanning cylinder 1. Likewise, the cells are engraved on parallel circumferential lines of the printing cylinder 2.

A prerequisite for the engraving of wells arranged without gaps on a circumferential line of the printing cylinder 2 is

that an even number of cells are engraved on this circumference. This condition is achieved by adjusting the speed of the cylinders.

Since the depth of a cell to be engraved is determined by the tonal value of the assigned image point on the image template 12, the number ZA of the image points scanned from the beginning to the end of an image line of the image template must equal the number ZD of the cells engraved on a closed circumferential line of the printing cylinder 2 be, the distance between two pixels corresponding to the distance between two cells. This condition is met by appropriate adjustment of the

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

627 406

4th

Frequency f3 of the sampling clock sequence T3 is reached with the aid of the divider stage 23.

The processes of scanning and engraving will be described in more detail below.

The first pixel of an image line is to be scanned on the upper edge 47 of the information-carrying part of the image template 12 and the last pixel on the lower edge 48 so that the edges of the image template 12 are not reproduced. In Fig. 1, the upper edge 47 is marked by an initial mark 160 on the scanning cylinder 1. The rotational speed of the cylinders and the frequency f3 of the scanning clock sequence T3 are selected such that exactly ZA pixels are scanned on an image line from the upper edge 47 to the lower edge 48 of the original 12.

With each cycle of the scan cycle sequence T3, that pixel is scanned which is currently in the optical axis of the scanning element 15 at the cycle time. The scanning clock sequence T3 must therefore be started such that at the clock time of the first clock of the scanning clock sequence T3 the first pixel is located on the upper edge 47 of the original 12 below the scanning element 15.

To solve this problem, a command “start of scanning” is obtained from a circumferential pulse derived from the rotary movement of the cylinders by a time delay. The command “start of sampling” then starts the sampling clock sequence T3 in a defined phase position.

A pulse generator 49 generates a circumferential pulse by photoelectric scanning of a mark 53 once per cylinder revolution. The mark 53 is arranged on the surface line 14, on which the register pins 13 are also located.

Since the optical axes of the pulse generator 49 and the scanning element 15 meet the surface of the scanning cylinder 1 on a surface line, the circumferential pulse is generated precisely when the register pins 13 are located under the scanning element 15. The pulse generator 49 is connected via a line 54 to an input 55 of a delay stage 56 in the synchronization stage 25. A second input 55 'of delay stage 56 is connected to the output of divider stage 10. In the delay stage 56, the command “start of scanning” is generated by delaying the circumferential pulse.

The time delay which is to be influenced via an actuating input 57 of the delay stage 56 must be set in such a way that the command “start of scanning” is given at the point in time at which the first pixel to be scanned is located under the scanning element 15.

The functioning of the synchronization stage 25 is as follows:

An output 58 of the delay stage 56 is connected to the set input of an RS flip-flop 59 and via an output 60 of the synchronization stage 25 to a reset input 61 of the divider stage 23. The Q output of the RS flip-flop 59 is connected to the other input of the AND gate 26. The “start of sampling” command appearing at the output 58 of the delay stage 56 first sets the RS flip-flop 59. The Q output of the RS flip-flop 59 is then in the H range and releases the AND gate 26 for the sampling clock sequence T3.

At the same time, the command “start of sampling” resets divider stage 23, so that the sampling clock sequence T3 always appears at the same phase at input 24 of synchronization stage 25 at this time.

With the “start of scan” command, the first clock of the scan clock sequence T3 reaches the A / D converter 17 via line 30 and there initiates the analog-to-digital conversion of the image signal into the data of the first pixel. These data, which are available at the digital outputs 20 of the A / D converter 17, are stored in the digital memory 22c of the image line storage device 22. The data of the following pixels of the first image line are also stored.

The data of the last pixel of the first image line are finally written into the digital memory 22c with the Zth clock of the scan clock sequence T3.

Then the write address counter 22a generates a command “scan end” at a signal output 62 of the image line storage device 22.

This “end of scan” command resets the RS flip-flop 59 via a line 63, an input 64 of the synchronization stage 25 and the reset input. The Q output of the RS flip-flop 59 enters the L region, and the AND gate 26 io is blocked for all further clocks of the scanning clock sequence T3, which ends the scanning process for the first image line.

Since the length from the upper edge 47 to the lower edge 48 of the information-carrying part of this image template 12 corresponds exactly to the circumference of the printing cylinder 2 in an exactly produced image template 12 and since the frequency f3 of the scanning clock sequence T3 is set such that the -formation-bearing part of the image template 12 Z pixels are scanned, the scanning process is ended exactly at the lower edge 48 of the image template 20 12 by the command “end of scan”.

To carry out a seamless engraving, the command “end of scanning” can be shifted with respect to the image template 12 in such a way that an exact seamless engraving is made possible even with image templates in which the distance between the edges 47 and 48 does not exactly correspond to the circumferential length 25 of the printing cylinder 2 . This method is described in detail with reference to FIG. 4. The "End of scan" command also controls the scan-side feed movement, after which the next image line is scanned. 30 The start of engraving on the printing cylinder 2 is determined by a command “start of engraving” which starts the recording cycle sequence T4 at a specific point in time and in a defined phase position. Each cycle of the recording cycle sequence T4 determines the time of engraving a well.

The command “start of engraving” is derived from the command “start of scanning” by a time delay in a further delay stage 65. The start of engraving on the impression cylinder can be varied by changing the time delay. This so-called “engraving” is described with reference to FIG. 3.

To obtain the “start of engraving” command, the output 58 of the delay stage 56 is connected to an input 66 of the delay stage 65. An output 67 of the delay stage 65 is fed to the set input of a further RS flip-flop 68 and via an output 69 of the synchronization stage 25 to a reset input 70 of the divider stage 38. The Q output of the RS flip-flop 68 is connected to the other input of the AND gate 40.

With the command “start of engraving”, the first cycle of the recording cycle sequence T4 appears at the clock input 43 of the read address counter 22b of the image line storage device 22 and directs the read process for the data of the first image point and the engraving of the first cell of the first circumferential line 55 a. After the engraving of a closed circumferential line, the recording is interrupted and the advancing movement of the engraving member is carried out in the direction of arrow 46. Then the engraving of the second circumferential line begins.

60 start of scan, end of scan and start of engraving are set by an operator changing the time delays for rotating cylinders. The setting must be made with sensitivity and reproducibility with the required register accuracy for the reproduction. A comparison device 163 serves to facilitate the settings 65, which is acted upon by the command “start of scanning” T7 on a line 162 and by the image signal T6 generated in the scanning element 15 on a line 161.

5

627 406

FIG. 2 shows an application example of the comparison device 163 for setting the start of scanning in the engraving system shown in FIG. 1. For the sake of clarity, only the assemblies that are essential for understanding the method have been adopted from FIG. 1. The image original 12 to be reproduced is spanned on the rotating scanning cylinder 1 and is scanned point by line and line by line during the reproduction to generate an image signal by the optoelectronic scanning element 15.

For example, the start of the scan may be fixed on the upper edge 47 of the information-carrying part of the image template 12. In this case, the first image point of an image line is scanned on the edge 47, with the result that the upper edge of the image original 12 is hidden during reproduction.

The start of the scan can of course be set as desired on the image template 12.

The desired start of scanning on the original 12 is marked by a mark 160 on the scanning cylinder 1.

During a test run to set the start of scanning, the scanning element 15 scans the mark 160 and generates a pulse T6 whenever the mark 160 moves under the scanning element 15.

During the test run, the pulse generator 49 emits a circumferential pulse by scanning the mark 53 with each cylinder revolution, which is fed via the line 54 to the delay stage 56 in the synchronization stage 25.

The delay stage 56 generates the pulse T7 “start of scanning” by delaying the circumferential pulse. The pulses T6 and T7 reach the comparison device 163 via lines 161 and 162.

By changing the time delay at the control input 57 of the delay stage 56 and with the aid of the comparison device 163, the pulses T6 and T7 are brought into agreement. In this case, the pulse T7 sets the RS flip-flop 59 and releases the sampling clock sequence T3 via the AND gate 26. At the same time, the pulse T7 resets the divider stage 23 via the reset input 61, with which the sampling clock sequence T3 is started in a defined phase position with the pulse T7 “sampling start”. This ensures that the first clock of the scanning clock sequence T3 appears exactly at each time and initiates the pixel scanning in each image line when the mark 160 is currently under the scanning element 15.

3 shows a further application example of the comparison device for setting the start of engraving when engraving an image template into printing cylinders that have already been partially engraved.

A first engraving 113 is visible on the impression cylinder 2 to illustrate the problem to be solved when engraving. An image template 114 for the first engraving 113 has already been removed from the scanning cylinder 1 and is therefore only shown in broken lines. The scanning cylinder 1 now carries an image template 115 for the post-engraving, which is to be carried out with registration accuracy on a field 116 of the printing cylinder 2 which was left free during the first engraving 113. Due to the reinstallation of the printing cylinder 2 for performing the post-engraving, the angular position of the cylinders with respect to one another that was present during the initial engraving has been lost, so that the cylinders are now rotated by a certain angle with respect to one another and the post-engraving would be incorrectly positioned offset by this angle to the first engraving.

To correct the inaccurate circumferential position of the cylinders relative to one another due to the renewed installation of the printing cylinder in the engraving machine, the pulse T8 “start of engraving” can be changed by an adjustable time delay compared to the circumferential pulse.

The time delay is carried out by means of a second delay stage 117 in the synchronization stage 25.

In order to determine the desired start of engraving, a mark 120 was applied to the printing cylinder 2 before the first engraving, on which the engraving of the first cell of an engraving line should begin.

During a test run, the mark 120 is scanned by a scanner 164, which always generates a pulse T9 when the mark 120 moves past under the scanner 164. This scanner 164 is arranged such that its optical axis hits the impression cylinder 2 on the same surface line as the engraving stylus tip of the engraving member 37.

The pulses T8 and T9 are fed via lines 161 'and 162' to a further comparison device 163 'assigned to the pressure cylinder 2.

With the aid of the second comparison device 163 ', the pulses Tg and T9 are brought into agreement by changing the time delay at a control input 118 of the delay stage 117. The pulse “start engraving” then sets the RS flip-flop 68 and releases the recording clock sequence T4 via the AND gate 40. At the same time, the pulse Ts “start of engraving” resets the divider stage 38 via the reset input 70, with which the recording cycle sequence T4 is started in a defined phase position with respect to the pulse Ts.

This ensures that the first stroke of the recording stroke sequence T4 appears at each cylinder revolution and initiates the well engraving when the mark 120 is just under the scanner 164 or the engraving stylus tip of the engraving member 37 below the marked start of engraving on the printing cylinder 2.

If this setting of the start of engraving is carried out both for the first engraving and for the post-engraving, all post-engravings are carried out with a precise registration in the circumferential direction to the first engraving.

FIG. 4 shows a further application example of the comparison device 163 for setting the start and end of scanning in seamless engraving.

For this purpose, an image original 12 is spanned on the scanning cylinder 1, the image content of which is identical at the upper edge 47 and at the lower edge 48. Due to manufacturing errors, however, the length between the edges 47 and 48 does not exactly correspond to the circumferential length of the printing cylinder to be engraved, so that exact seamless engraving is not possible per se. The possibility of setting the start and end of the scan, however, makes it possible to produce an exact endless pattern even if the dimensions of the image original 12 were not exactly adhered to.

Compared to the arrangement for setting the start of scanning described in FIG. 1, the circuit according to FIG. 4 is expanded by a counting circuit 121 and an OR gate 122. The frequency divider 23 is replaced by an adjustable frequency divider 123, with which the sampling clock sequence T3 can be varied in fine frequency f3. In delay stage 56, pulse T7 “start of sampling” is again obtained by delaying the circumferential pulse. The pulse T7 reaches the comparison device 163 via a line 134, the OR gate 122 and a line 167 and via a line 124 to the clock input of a flip-flop 125 in the counting circuit 121.

The Q output of the flip-flop 125 is led to an input of an AND gate 126, at the second input of which the sampling clock sequence T3 arrives via a line 127 and whose output is connected to the counter input 128 of a counter 129. An output 130 of the counter 129 is connected via a line 131 and via the OR gate 122 to the comparison device 163 and via a line 132 to the reset input of the flip-flop 125. The counter 129 is preset via a programming input 133 to the number (ZA + 1) of pixels to be scanned.

The counter circuit 121 generates at (ZA + 1) after

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

627 406

6

The pulse T7 counted clocks of the sampling clock sequence T3 a further pulse T12 "end of sampling", which is also forwarded to the comparison device 163 via the line 131, the OR gate 122 and the line 167.

The beginning and end of scanning on the original 12 are marked by an initial mark 165 and an end mark 166 on the scanning cylinder 1, which are scanned by the scanning element 15 during a test run and two consecutive pulses Tjo and Tn ran. The pulses T10 and Tu are fed to the comparison device 163 via the line 168. To set the start of the scan, as described in FIG. 1, the pulse T7 “scan start” and the pulse T10 assigned to the mark 165 are brought into agreement by changing the time delay at the control input 57 of the delay stage 56 with the aid of the comparison device 163.

To set the scanning end, the frequency f3 of the scanning clock sequence T3 on the frequency divider 123 is varied until the pulse T12 “scanning end” and the pulse Tn assigned to the mark 166 are also congruent. Then the condition for engraving a seamless endless pattern on the circumference of the printing cylinder 2 is fulfilled.

5 shows an exemplary embodiment of the comparison device 163 in the form of an optical observation arrangement for determining the coincidence of the pulses. The pulses first arrive at a pulse shaping stage 169 in order to obtain the same pulse width for both pulses. The pulse width is selected according to the permitted tolerance when setting the coincidence. The pulse shaping stages 169 are connected to the vertical inputs of an oscillograph 171, on the fluorescent screen 172 of which the pulses are in two. Traces as line marks 173 are made visible. By changing the time delay of the circumferential pulse, one of the line marks 173 can be shifted in the horizontal direction. The coincidence of the impulses is reached when both line marks 173 lie exactly on a vertical line.

6 shows a further exemplary embodiment of the comparison device 163 in the form of an electronic coincidence circuit for the pulses. The pulses to be compared first arrive at a pulse shaping stage 174 in order to bring the pulses to the same pulse width and to TTL level 15. The pulse shaping stages 174 are connected to the inputs of an AND gate 175, at the output of which an H signal appears when the pulses coincide.

Of course, any other form of coincidence circuit can be used.

20 The AND gate 175 can be connected to a signaling device which provides a visible or acoustic display when the H signal appears.

The AND gate 175 could also be in operative connection with the delay stages 58 and / or 117 in the synchronization stage 25. Then the output signal of the AND gate 175 would automatically start and stop the time delay of the circumferential pulse.

s

5 sheets of drawings

Claims (7)

627 406 1. A method for adjusting the reproduction of an image template in a precise location, in which the image template (12) spanned on a scanning cylinder (1) is scanned point by point and image line by pixel by an optoelectronic scanning element (15) to obtain an image signal and in which the reproduction is carried out on a recording cylinder (2) is carried out by a recording element (37) controlled by the image signal, a circumferential mark (53) determining the start of scanning being provided on the scanning cylinder (1), characterized in that by scanning a desired start of scanning on the image original (12) determining starting mark (160; 165), a first signal is generated that a second signal "start of scanning" is derived by a variable time delay device (56) from a circumferential pulse, which in each case at a specific angular position of the scanning cylinder (1) by the circumferential mark ( 53) is generated, and that for setting the start of scanning on the B Image template for later reproduction, the signals during a test run can be matched by means of a comparison device (163) when the signal “start of scanning” appears by changing the time delay device. 2. The method according to claim 1, characterized in that the signals are matched with an electron beam tube (172) by means of an optical comparison device (Fig. 5). 2nd PATENT CLAIMS 3. The method according to claim 1, characterized in that the signals are matched by means of an electronic comparison device with a coincidence circuit (175) (Fig. 6). 4. The method according to any one of claims 1 to 3, characterized in that the image signal is digitized and the data thus obtained are converted back into an analog image signal, the digitization being controlled by a sampling clock sequence (T3), and that the signal “start of sampling” Sampling clock sequence starts in a defined phase position. 5. The method according to claim 4, characterized in that as many clocks of the sampling clock sequence (T3) are counted (22a) as pixels are scanned from an image line, and that a signal “end of scanning” is generated in each case at the last counted clock, that interrupts the sampling clock sequence. 6. The method according to claim 5, for seamless engraving, characterized in that the digital image signal is written by the scanning clock sequence (T3) into a buffer (22c) and is read out again by a recording clock sequence (T4), that by scanning one of the scanning end on the original image (12) determining the final mark (166) a fourth signal (Tu) is generated and that the fourth signal and the signal "sampling end" (T12) by means of the comparison device (163) by changing the frequency of the sampling clock sequence when the signal "sampling end" appears Be matched. 7. The method as claimed in claim 4 or 5, for engraving, characterized in that the image signal is written into a buffer (22c) by the sampling clock sequence (T3) and is read out again by a recording clock sequence (T4), the recording clock sequence also generating the printing pattern, that a further signal (T9) is generated by scanning by means of a further scanning element (164), a further initial mark (120) determining a desired start of engraving on the recording cylinder (2), the last-mentioned scanning element (164) and the recording element (37) in their circumferential position to the recording cylinder (2) agree that a signal «start of engraving» (Ts) is obtained from the signal «start of scanning» by a variable time delay, and that to set the start of engraving for later reproduction, the further signal mentioned (T9) and the signal “start of engraving” (T8) by means of a comparison device (163 ') during a Pr above by changing the time delay when the "Engraving start" signal appears and the "Engraving start" signal starts the recording cycle in a defined phase position.